1. Field of the Invention
The present invention relates to an on-demand inkjet head and an inkjet recording apparatus on which the inkjet head is mounted.
2. Description of the Related Art
There has been known an on-demand inkjet head for changing a pressure in a pressure generating chamber in which ink is charged by applying a voltage to a piezo-electric member, and discharging an ink drop from an opening of a nozzle communicating to the pressure generating chamber. However, it has been difficult to increase the printing speed while enhancing the stability of the ink discharge operation by the inkjet head of this type. Here, the stability of the ink discharge operation means a property that a variation in the speed of an ink drop to be discharged or a volume of an ink drop to be discharged is small.
In order to keep stabilization of the ink discharge operation, it is required that the variation of the meniscus position of the ink in the nozzle is reduced and the meniscus position is stabilized in the vicinity of the opening of the nozzle when the ink discharge operation is started.
On the other hand, a frequency of the ink drop to be discharged has only to be increased in order to increase the printing speed. In order to increase the drive frequency of the ink drop to be discharged, it is required that the speed at which the meniscus retracted by the ink discharge operation is returned to the original position, that is a meniscus return speed is improved. However, when the meniscus return speed is improved, the meniscus overshoots from the opening of the nozzle due to inertia of an ink flow along with the return of the meniscus. Therefore, the meniscus position is easily unstable in the vicinity of the opening of the nozzle. When the ink discharge operation is started in a state where the meniscus position is unstable, the discharge speed or the discharge volume is fluctuated, or the ink not-discharged phenomenon occurs in some cases, so that the stability of the discharge operation is easily lost. In this manner, it has been difficult to achieve both the stability of the meniscus position and the improvement of the meniscus return speed.
In order to solve such problems, there is disclosed (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-117972) a technique where, assuming that a relationship between the properties of the ink and the shape of the ink flow passage is prescribed and the maximum drive frequency is 10 kHz in order to realize the target printing speed, both the stability of the meniscus position and the improvement of the meniscus return speed can be achieved even when the environment temperature changes.
However, in the conventional technique disclosed in this patent reference, it becomes clear from a simulation by the present inventor that, when the target maximum drive frequency is made higher than 10 kHz, overshooting of the meniscus largely occurs.
In other words, the present inventor has performed the simulation of an operation for discharging one ink drop using the following numerical values as characteristic values in the numerical range indicated in this conventional technique.
Total inertance mT=9.8×107 [kg/m4]
Total acoustic resistance rT=6.7×1012 [Ns/m5]
Surface tension of ink=30 [mN/m]
When a variation in the meniscus position after the completion of the ink discharge operation is found by this simulation, a result indicated by a solid line P in FIG. 12 is obtained.
A meniscus volume position v(t) in FIG. 12 is a value where a position of the meniscus is expressed by a volume. As shown in FIG. 13A, when the meniscus of ink 1 is retracted from an opening 2a of a nozzle 2, a volume Vi of air in the opening 2a of the nozzle 2 is assumed to be a negative value of the meniscus volume position. Further, as shown in FIG. 13B, when the meniscus of the ink 1 is advanced from the opening 2a of the nozzle 2, a volume Vo of the ink which is projected from the opening 2a of the nozzle 2 is assumed to be a positive value of the meniscus volume position.
In FIG. 12, dotted lines S1 and S2 indicate an allowable range of the meniscus volume position v(t) which does not affect the operational stability when the next ink discharge operation is started. In the case of the printing condition generally used, when the allowable range is ±5% relative to the discharge volume, the discharge stability can be obtained. Here, the grounds for ±5% is based on a numerical range where those skilled in the art regard allowable limits that image quality is not deteriorated.
Therefore, as can be seen from FIG. 12, in the inkjet head disclosed in this conventional technique, the overshooting of the meniscus after ink is discharged is large, and a time until the variation in the meniscus falls into the prescribed allowable range, that is the meniscus return time is long. Thus, it is difficult to improve the drive frequency for discharging ink while keeping the stabilization of the ink discharge operation.
In the meantime, there has been conventionally known a technique for continuously discharging a plurality of small ink drops as a technique for performing gradation printing (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2002-19103). The present inventor applies this technique to the inkjet head of the conventional technique and performs a simulation of the discharge operation when seven ink drops correspond to the maximum dot diameter in gradation printing are continuously discharged to find a variation in the meniscus position after the completion of the ink discharge operation. Therefore, a result indicated by a double-chain line Q in FIG. 12 is obtained.
As shown in FIG. 12, when a plurality of small ink drops are continuously discharged, the meniscus return speed is faster as compared with a case where only one ink drop is discharged. Thus, the overshooting of the meniscus after ink is discharged is more pronounced than that in the case where only one ink drop is discharged. Therefore, when a plurality of small ink drops are continuously discharged to perform gradation printing, it is further difficult to reduce the meniscus return time.
As described above, in the conventional inkjet head of this type, is has been difficult to increase the printing speed, that is, to discharge ink at a high drive frequency, while enhancing the stability of the ink discharge operation.